.alpha.-Alumina powder is widely used as a raw material for abrasives, sintered products, plasma spraying materials, fillers, etc. .alpha.-Alumina powder obtained by conventional processes which have been generally employed comprises irregular-shaped polycrystals, contains many agglomerated particles, and has broad particle size distribution. Furthermore, the purity of such conventional .alpha.-alumina powder is insufficient for some uses. In order to overcome these disadvantages for some specific uses, .alpha.-alumina powder produced by special processes as hereinafter described has been employed. However, these special processes still involve difficulty in producing .alpha.-alumina powder with a narrow particle size distribution, which comprises homogeneous .alpha.-alumina particles.
Among general processes for producing .alpha.-alumina powder a Bayer's process is the most economical process. In a Bayer's process, bauxite is once converted to aluminum hydroxide or transition alumina, which is then calcined in air to prepare .alpha.-alumina powder.
The aluminum hydroxide or transition alumina which is obtained as an intermediate product on an industrial scale at low cost generally comprises agglomerated particles having a diameter of greater than 10 .mu.m. .alpha.-Alumina powder obtained by calcination of such aluminum hydroxide or transition alumina in air comprises primary particles of irregular shape containing many coarse particles agglomerated strongly. The .alpha.-alumina powder containing coarse agglomerated particles is ground into final products by means of a ball mill, a vibration mill, etc., but grinding is not always easy and incurs the cost. Further, .alpha.-alumina powder having poor grindability needs a long grinding time. Consequently too fine powder may be formed, which is unsuitable particularly as a raw material of abrasives.
Several proposals have been made to solve these problems. For example, JP-A-59-97528 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") teaches a process for improving the shape of .alpha.-alumina powder, which comprises calcining aluminum hydroxide prepared by a Bayer's process in the presence of an ammonium-containing boron or boron series mineralizer to obtain .alpha.-alumina powder.
In order to remove sodium and to control the particle size simultaneously, it has been proposed to carrying out calcination of sodium-containing aluminum hydroxide obtained by a Bayer's process in the presence of (1) a chlorine-containing material and a fluorine-containing material (as described in British Patent 990,801) or (2) boric acid and a chloride, such as ammonium chloride or hydrogen chloride (as described in West German Patent 176,511).
JP-B-43-8929 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses calcination of alumina hydrate in the presence of ammonium chloride.
However, any of these processes proposed is still unsatisfactory for obtaining homogeneous and high-purity .alpha.-alumina powder having a uniform particle shape and a narrow particle size distribution.
Known special processes for producing .alpha.-alumina powder include a hydrothermal process comprising a hydrothermal reaction of aluminum hydroxide in the presence of corundum as a seed crystal (as described in JP-B-57-22886) and a flux process comprising calcining aluminum hydroxide in the presence of a fluorine series flux having a melting point of 800.degree. C. or lower (as described in JP-A-3-131517).
Because the hydrothermal process is carried out in a high temperature under a high pressure, it involves a problem in that the resulting .alpha.-alumina powder becomes expensive. Because the flux added remains in the .alpha.-alumina powder obtained by the flux process, it involves a problem in that the remaining flux must be removed by washing to obtain .alpha.-alumina of high purity.